This invention relates to a method of attaching the final steam turbine bucket (also called the closure bucket) of a row of buckets on a turbine rotor or wheel.
Steam turbine blades, or buckets, are often designed for installation on a turbine wheel in a tangential direction. The buckets are typically attached to the turbine wheel using external circumferential dovetails, with a male dovetail on the wheel periphery and a complimentary female dovetail in the base or root of the bucket. In order to load these buckets onto the wheel, a notch which locally removes the male dovetail portions is cut on the periphery of the wheel, leaving a generally rectangular core portion. Each bucket is then initially located over the core material in the notch and then displaced tangentially onto and around the wheel. Once all the buckets have been loaded, a closure block is utilized that is formed with laterally spaced tangs extending radially inwardly and that are adapted to straddle the core material in the notch. The closure block is secured by a retaining pin passing through the tangs and core. In this way, the buckets on the wheel are locked in place and thus prevent the buckets from moving circumferentially along the dovetail.
Front or first stage turbine buckets are subjected to high temperatures over 900xc2x0 F. Limitations of material stress capability mean that only a lightweight block, which has no airfoil, can be used as the closure block, causing reduced performance. Because the closure block has no airfoil, there is an opening in the steam path with detrimental effects on performance. The reason behind the inability to support an airfoil on the closure bucket is the fact that the retaining pin passes through the core material in the highly stressed dovetail region of the wheel. There is thus a need for a first stage closure block with a mounting or retaining arrangement that provides sufficient strength to permit the incorporation of an integral airfoil that closes the opening, thus producing greater performance.
The new closure bucket design in accordance with this invention has two longer tangs which fit into a radially extended loading notch in the wheel. Two axial pins pass through the tangs and a remaining core portion of the wheel, but radially inside the dovetail region. This design has the effect of placing the pins in a location in the wheel where the stress level is reduced, thus allowing the addition of an airfoil in the opening over the closure block.
In its broader aspects, the invention relates to a closure bucket for a first stage turbine wheel comprising a root portion, a platform and airfoil, the root portion formed with a pair of radially inwardly extending laterally spaced tangs on either side of the root portion, the tangs each formed with a pair of radially aligned retaining pin holes.
In another aspect, the invention relates to a turbine wheel having a male dovetail configuration about substantially an entire periphery of the wheel, interrupted by a notch formed by removal of portions of the male dovetail at a bucket loading location on the periphery of the wheel; and a closure bucket comprising a root portion, a platform and airfoil, the root portion formed with a pair of radially inwardly extending laterally spaced tangs each formed with a pair of radially aligned retaining pin holes, the retaining pin holes located radially inwardly of the male dovetail configuration.
The invention will now be described in greater detail in connection with the drawings identified below.